1. Field of the Invention
The present invention relates generally to methods of generating image data, image data generation apparatuses, and image formation systems, and particularly to methods of generating image data, image data generation apparatuses, and image formation systems, that embed information in image data.
2. Description of the Related Art
Some electronic watermarking technique for documents are known as those embedding information, such as data indicating a copyright holder, in a background of an original image of characters and the like in the form of a woven pattern.
For example, the present inventor has earlier filed some patent applications, now published as US2006/0262957A1 and Japanese Laid-Open Patent Publication No. 2003-283797, which disclose techniques using a plurality of point dots and depending on whether a dot is present or absent to assign 0 (dot absent) and 1 (dot present) to embed information.
Furthermore, as another method, Japanese Patent No. 3837999 and Japanese Laid-Open Patent Publication No. 2004-080136 disclose techniques assigning 0 and 1 to two types of linear dots inclined in different orientations, respectively, to embed information.
It should be noted herein that a dot formed of two-dimensionally uniformly disposed pixels will be referred to as a “point dot” and a dot formed of one-dimensionally disposed pixels will be referred to as a “linear dot”. Through human eyes, the dots are both seen as small spots, and they will be referred to herein as dots.
US2006/0262957A1 and Japanese Laid-Open Patent Publication No. 2003-283797 disclose techniques employing only one type of point dot. As such, they are characterized in that they can facilitate detecting embedded data. However, there exists a location without a dot, and in comparison with embedding dots equidistantly, an image having undergone embedment has an unintended pattern caused therein. This provides poor appearance and as a result the original image is poorly visibly recognized.
The techniques disclosed in Japanese Patent No. 3837999 and Japanese Laid-Open Patent Publication No. 2004-080136 embed dots equidistantly and will thus not cause an unintended pattern. However, they require a process in detecting embedded data for determining an orientation of a linear dot, and it is thus difficult to detect data.
Image formation apparatuses provide reproducibility for lines, as will be described hereinafter. Image formation apparatuses form an image by various image formation methods, and electrophotography employing a laser to write on a photoreceptor, a serial head ink jet system, and the like are known as such methods. Electrophotography employing a laser causes the laser to scan to form an electrostatic latent image on a photoreceptor and develops the electrostatic latent image to provide a tonered image which is in turn transferred on a sheet. In the serial head ink jet system, a sheet is fed, while an ink jet head is moved in a direction orthogonal to that in which the sheet is fed to form an image on the sheet. Image formation apparatuses adopting these systems write in a main scanning direction in an order. As such, they exhibit a varying performance in terms of function that joins pixels together more readily in the main scanning direction and less readily in a vertical scanning direction. Note that the vertical scanning direction is a direction corresponding to that in which a sheet (a recording medium) is fed and the main scanning direction is a direction orthogonal to the vertical scanning direction. An image formation apparatus also forms an image by a known image formation method of a line head system having a plurality of recording heads disposed in the main scanning direction. For example, electrophotography employing a light emitting diode (LED) line head employs a plurality of LEDs staggered in the main scanning direction to write on a photoreceptor. Furthermore, a line head ink jet system employs a plurality of ink jet heads staggered in the main scanning direction to write on a sheet in the main scanning direction collectively. These line head system image formation apparatuses have a varying performance in terms of function that more readily joins pixels in the vertical scanning direction and less readily joins pixels in the main scanning direction because of the staggered arrangement in the main scanning direction.
Accordingly, the techniques disclosed in Japanese Patent No. 3837999 and Japanese Laid-Open Patent Publication No. 2004-080136 provide a less stable output for a linear dot having some geometry (or orientation). As a result, a less detectable linear dot is generated, and embedded data is detected with poor precision.